(1) Field of the invention
The present invention relates to a system and method for controlling an air/fuel mixture ratio for an internal combustion engine which specially carry out fault diagnoses for a secondary air supply apparatus located in an exhaust gas passage upstream of an oxygen concentration sensor.
(2) Description of the background art
In an exhaust gas passage of an internal combustion engine, a three-way catalytic converter is installed. To improve conversion efficiencies of respective exhaust gas components (CO, HC, and NOx), an air/fuel mixture ratio feedback control is carried out so that an air/fuel mixture ratio on the exhaust gas passing through the catalytic converter falls in a narrow range with a stoichiometric air/fuel mixture ratio as a center. However, in order to improve a driveability of the engine, the air/fuel mixture ratio feedback control is halted.
A case where the driveability of the engine taken into consideration includes an engine cooling situation. In this case, since a fuel combustion is not stable, the air/fuel mixture ratio feedback control is halted and, in place of it, a richer air/fuel mixture ratio with respect to the stoichiometric air/fuel mixture ratio with a correction of the air/fuel mixture ratio has been made to increase the fuel injection quantity according to a temperature of an engine coolant so as to stabilize engine revolutions.
However, the internal combustion engine is often provided with a secondary air supply apparatus to introduce a secondary air to the exhaust gas passage in order to prevent the conversion efficiencies of HC and CO from being reduced when the exhaust gas passing through the catalytic converter installed in a midway through the exhaust gas passage becomes richer air/fuel mixture ratio due to the increased quantity of fuel. Therefore, since the secondary air is introduced into a part of the exhaust gas passage upstream of the oxygen concentration sensor (so-called O.sub.2 sensor) so that the exhaust gas is returned to a leaner air/fuel mixture ratio with respect to the stoichiometric air/fuel mixture ratio or near to the stoichiometric air/fuel mixture ratio so as to promote oxidation of HC and CO and so as to increase an exhaust gas temperature by burning an uncombusted HC, thus quickening to activate the catalytic converter.
If, in this case, a failure in the secondary air supply apparatus occurs, the conversion efficiencies of HC and CO can be reduced.
A Japanese Patent Application First Publication No. Showa 63-111256 published on May 16, 1988 exemplifies a previously proposed secondary air supply apparatus failure diagnosing system.
In the above-identified Japanese Patent Application First Publication, a system for controlling air/fuel mixture ratio determines an occurrence in failure in the secondary air supply system when a signal output from the oxygen concentration sensor indicates a richer air/fuel mixture ratio even during the introduction of secondary air to the exhaust gas passage. This is because the failure in the secondary air supply system is caused by a reduced flow quantity of the secondary air from an air pump or by an open/close valve installed in a passage of the secondary air supply apparatus which is stuck to a full closure position or which does not open sufficiently. Consequently, an insufficient quantity of the secondary air is resulted and the exhaust gas provides and maintains the air/fuel mixture ratio at a richer air/fuel mixture ratio.
However, an accuracy of the determination of failure of the secondary air supply apparatus becomes reduced since the oxygen sensor itself outputs the richer air/fuel mixture ratio signal due to the failure in the oxygen concentration sensor in the case of the air/fuel mixture ratio control and secondary air supply failure diagnosing apparatus disclosed in the above-identified Japanese Patent Application First Publication.
For example, when an intake air quantity characteristic becomes deviated toward a larger airflow quantity due to variations in the intake air flow quantity measured by an airflow meter and due to its aging effect and a larger quantity of fuel is injected through a fuel injection valve according to the increased intake air flow quantity, the air/fuel mixture ratio is determined to be richer if it is at the air/fuel mixture ratio feedback control so that the air/fuel mixture ratio feedback control system is operated to decrease a basic fuel injection pulsewidth Tp with the air/fuel mixture ratio feedback correction coefficient. Consequently, the air/fuel mixture ratio does not tend to be directed toward the richer air/fuel mixture ratio.
However, while the secondary air is introduced, the air/fuel mixture ratio feedback control is halted so that the decreasing correction by means of .alpha. does not work. Therefore, due to larger quantity of fuel injected through the fuel injection valve accompanied with an erroneous detection of the airflow meter, the air/fuel mixture ratio tends to be directed toward relatively richer side, thus being detected by means of the oxygen concentration sensor.
However, even, at this time, the air/fuel mixture ratio control and secondary air supply apparatus diagnosing systems determine the failure in the secondary air supply apparatus since the output signal of O.sub.2 sensor indicates the rich state during the introduction of the secondary air to the exhaust gas passage.
On the contrary, the previously proposed system disclosed in the above-identified Japanese Patent Application First Publication does not determine the failure in the secondary air supply apparatus even when the secondary air supply apparatus actually fails. For example, when the airflow meter detects the intake air quantity less than the actual intake air quantity during, e.g., a transient operating condition or the flow quantity characteristic is maintained at the stoichiometric air/fuel mixture ratio or deviated toward a leaner side even when the flow quantity of the secondary air is appropriate.